This invention relates to mechanisms for changing gear speed ratios between selective values. More particularly, this invention relates to speed change mechanisms including gear shift yokes actuated by travelling nuts on screw shafts.
Prior art devices of the type referenced herein are subject to frequent failures resulting from premature wear. Such devices utilize nuts which contain rolling balls juxtaposed between the nut body and grooves of the screw shaft. The balls allow for a slight and desirable amount of overrunning of the nut at the ends of stroke travel. However, the balls tend to make point (rather than surface) contact with the grooves of the screw shaft resulting in accelerated or premature wear of both balls and shaft. As the balls decrease in size, contact stresses become even greater. Further deterioration results in failure of the screw assembly to traverse, which is ultimately followed by motor failure.